I've found similar question name, but it is a little different.
Office365 UI interface can provide recurring rule where repeat: monthly On second weekday or repeat: monthly On third weekend day
How I could represent second weekday and third weekend day using RFC 5545?
monthly on second weekday:
FREQ=MONTHLY;BYDAY=MO,TU,WE,TH,FR;BYSETPOS=2
This rule expands to every weekday of each month, but in the end selects only the 2nd day of the set.
See the expanded result: http://recurrence-expansion-service.appspot.com/reaas?dtstart=20170602T000000&rrule=FREQ%3DMONTHLY%3BBYDAY%3DMO%2CTU%2CWE%2CTH%2CFR%3BBYSETPOS%3D2&skip=&max_instances=100&expansion_window_end=21000101T000000&strict=1
monthly on third weekend day
FREQ=MONTHLY;BYDAY=SA,SU;BYSETPOS=3
This rule expands to every weekend day of each month, but in the end selects only the 3rd result of the set.
See the expanded result: http://recurrence-expansion-service.appspot.com/reaas?dtstart=20170610T000000&rrule=FREQ%3DMONTHLY%3BBYDAY%3DSA%2CSU%3BBYSETPOS%3D3&skip=&max_instances=100&expansion_window_end=21000101T000000&strict=1